Method for event-related determination of process variables

ABSTRACT

Process variables (s 1 , s 2 ) are detected and processed cyclically in a controller (BG). Process variables (s_(x)) from the adjacent cyclically determined process variables (s_(n−1), s_(n)) are determined as being event-related as a function of the time (t_(x)) of the event within the cycle. The calculation also includes derivatives of the process variable (s).

FIELD OF THE INVENTION

[0001] The present invention relates to a method for event-relateddetermination of process variables in control assemblies, in particularmicrocomputer-aided control assemblies, in which process variableprofiles are detected by cyclic sampling, and processed in a controlprogram. Such methods are generally well-known and used in controlassemblies for machines of all types.

BACKGROUND OF THE INVENTION

[0002] Microcomputer-aided control assemblies in which process variablessuch as distance, pressure, temperature etc are processed are used forcontrolling machines, handling devices and system parts. In general,assemblies such as these operate cyclically, in a timed mode. In thiscase, it is necessary to record the process variables as they exist atthe sampling time, for which purpose the assemblies have parts whichcontain memories in which the instantaneous values of the processvariables are stored at the sampling time either directly by the clockor in the context of an interrupt service routine. The control programthen refers to the instantaneous values for open and closed-loop controlpurposes in the time period between two samples.

[0003] For certain control tasks, it is necessary to detectinstantaneous values of process variables at the time of a processevent, for example the position value of a machine shaft at theswitching time of a measurement probe. Parts containing additionalmemory are provided for this purpose, in which the instantaneous valuesof the process variables are stored with respect to a signal whichindicates the process event. These parts containing additional memoryare required for each process variable and each process event.

[0004] Control engineering concepts in which the components of a systemare arranged in a decentralized manner and are coupled to one anotherhierarchically via a bus system are being used increasingly. In thiscase, the process variables are detected using different components,which are synchronized by suitable mechanisms beyond the bus. However,such mechanisms are not suitable for transmitting the switching signalof a process event precisely at the event time through the bus to thecomponents which require the switching signal. The switching signaltherefore has to be passed to these components via an additionalconnecting line.

[0005] It is also possible for a switching signal which indicates theprocess event to be supplied to a central memory, which stores theactual time of the process event, with, this time being relatedabsolutely or relatively to the sampling times (of which the controlprogram is aware) and with the control program using the instantaneousvalues of the process variables of the samples on either side of thetime of the process event to calculate the magnitude of the processvariable corresponding to the time of said process event. However, thissolution has the disadvantage that the calculated value has an errorfrom the actual value from the actual profile of the process variablebetween the sampling times, at the time of the process event. Themagnitude of this discrepancy differs, depending on the time intervalbetween the samples and the relationship with the process variabledynamics. The object of the present invention is to provide thecalculated value of the process variable with high accuracy.

SUMMARY OF THE INVENTION

[0006] According to the present invention, this object is achieved by amethod in which the time t_(x) of a process event is related absolutelyor relatively to the preceding or subsequent cyclic sampling timet_(n−1) or t_(n) and the control program determines the instantaneousvalue, which corresponds to the time t_(x) of the process event, of aprocess variable p_(x). This determination is made by a calculation fromthe instantaneous values p_(n−1) or p_(n) of the process variable fromthe samples on either side of the process event, and in that thevariable state of the first and second derivations of the processvariable p are taken into account when determining the process variablep_(x) for the time t_(x) of the process event. The application for thepresent invention as originally filed in German is incorporated hereinby reference.

[0007] A preferred embodiment of the present invention is obtained bytaking account of the third derivative of the process variable p whendetermining the variable state. The method can be carried out easily inthat the process variable p_(x) for the time t_(x) of the process eventis determined with reference to the value of the variable state p whichresults from the closest sample t_(n−1) or t_(n) to the time t_(x) ofthe process event.

[0008] However, as an alternative, it is also possible in the method asinitially disclosed for the time t_(x) of a process event to be relatedabsolutely or relatively to the preceding or subsequent cyclic samplingtime t_(n−1) or t_(n), and wherein the control program determines theinstantaneous value, which corresponds to the time of the process event,of a process variable p_(x) by calculation via a ratio assessment. Thiscalculation takes into account the relative position of the time of theprocess event t_(x), from the instantaneous values p(n−1) or p_(n) ofthe process variable at the time of the samples on either side of theprocess event. There is therefore no need either for any partscontaining additional memories in the control assemblies or for anyadditional connecting lines for the switching signal relating to theprocess event between components arranged in a decentralized manner.Furthermore, since the process variable p is detected in componentswhich are coupled by means of a bus system, decentralizedmulti-component operation is feasible.

DRAWINGS

[0009] Exemplary embodiments of the present invention are disclosed ingreater detail in the following text, and are illustrated in thedrawings, in which:

[0010]FIG. 1 shows a block diagram of a positioning assembly forcontrolling machine shafts;

[0011]FIG. 2 shows a distance/time diagram;

[0012]FIG. 3 shows a correlation map; and

[0013]FIG. 4 shows a process variable profile.

DETAILED DESCRIPTION OF THE INVENTION

[0014]FIG. 1 shows a block diagram of a positioning assembly BG forcontrolling machine shafts which is coupled via a bus to a number ofdrives A1, A2, etc. All the drives A1, A2, etc. are synchronized via aglobal control message as slaves to the cycle clock CT of thepositioning assembly BG which is the master. The position values S1, S2,. . . of the shafts in the drives A1, A2, . . . are detected at the timeof this clock pulse, and are then transmitted sequentially via the busto the positioning assembly BG, where they are stored in memories SP1,SP2 as timed position values s1_(n), s2_(n) for a control program PROG.The control program PROG thus operates with position values s1_(n),s2_(n) which correspond exactly to the values of the process variable sat the respective sampling time t_(n). The position values s_(n) arestored in the control assembly as s_(n−1), s_(n−2), to a depth coveringa number of samples.

[0015] The control program PROG calculates the variable state velocityv_(n) from the respective position values s_(n) and s_(n−1) for eachmachine shaft in the sampling cycle, and uses v_(n) and v_(n−1) tocalculate the variable state acceleration a_(n). The variable statev_(n) in this case applies to the relative time precisely in the centerbetween two samples t_(_(m))=0.5*(t_(_(n))−t_(_(n−1))). The variablestate a_(n) applies in a corresponding manner to the time t_(n−1).

[0016] An object of the positioning assembly BG is to detect the contactpoint of a measurement probe MT with a tool or a workpiece when saidmeasurement probe MT is deflected. To this end, a measurement probesignal MTS is passed as a transfer call to a memory SP3 in thepositioning assembly BG. Memory SP3 stores the instantaneous value of atimer ST which is started with the initial value 0 with each clock cycleof a central clock transmitter CT in the positioning assembly BG. At thetime when the measurement probe MT switches, the time t_(x) istransferred from the timer ST to the memory SP3. In the following clockcycle t_(n), the process event is identified and, in the clock cyclet_(n+1), the position values s_(x) which are associated with theswitching time t_(x) of the measurement probe MT are determined from theavailable variables for the samples (n−1) to (n+1) for one or moreshafts, using the following relationship: if ( t_(x)>t_(m) ) {//reversecalculation from cycle t_(n) delta_t=t_(n+1) − t_(n) − t_(x);s_(x)=s_(n) − ( 0.5 * (v_(n)+v_(n+1) ) − a_(n+1) * delta_t) * delta_t}else {//forward calculation from cycle t_(n−1) delta_t=t_(x);s_(x)=s_(n−1)+( 0.5 * ( v_(n−1)+v_(n) ) + a_(n) * delta_t) * delta_t}

[0017] In the same way, other process variables can be determined on anevent-related basis in other applications, for example pressure ortemperature values, which are processed in a user program in a PLC(programmable logic controller). One advantage of the present inventionis that it is also applicable to arrangements which are notdecentralized in that standard assemblies can be used which have nomemories for event-dependent storage of instantaneous values of aprocess variable.

[0018]FIG. 2 shows the profile of any given position value s which may,for example, correspond to the position value s1, plotted against thetime t, to be precise including the timed and calculated variables. FIG.3 correlates the time t, position values s, velocity v and accelerationa. FIG. 4 shows the profile of a process variable p which may, forexample, correspond to a position value s, plotted against time tincluding the timed and calculated variables. The graphically visualizeddetermination option for the value of the process variable and the timet_(x) is in this case assumed to be a simple linear interpolation,namely:

(t_( x)−t_( n−1))/(t_( n)−t_( n−1)) corresponding to (p_( x)−p_(n−1))/(p_( n)−p_( n−1)).

[0019] The value of the process variable p_(x) is thus calculated as:

p_( x)=p_( n−1)+(P_( n)−p_( n−1))*(t_( x)−t(n−1))/(t_( n)−t_( n−1).

[0020] In a control assembly in which a number of process variables areprocessed, such as the position values s1, s2 and s3 in athree-dimensional coordinate system, the use of this calculation methodmakes it possible to determine the three-dimensional coordinates(s1_(x); s2_(x); s3_(x)), in which case a common time (t_(x) of theprocess event is used as a reference for all three process variables.

[0021] In this way, the method is generally applicable to any desirednumber of process variables, and to physically different processvariables.

We claim:
 1. A method for event-related determination of processvariables in control assemblies comprising detecting process variableprofiles by cyclic sampling, and processing said variable profiles in acontrol program, wherein a process event time is related to a cyclicsampling time, and wherein the control program determines aninstantaneous value, which corresponds to the time of the process event,of a process variable by calculation from the instantaneous value of theprocess variable from samples on either side of the process event, andwherein variable states of a first and second derivative of the processvariable are taken into account when determining the process variablefor the time of the process event.
 2. The method according to claim 1,wherein the third derivative of the process variable is taken intoaccount when determining the variable state.
 3. The method according toclaim 1, wherein the process variable for the time of the process eventis determined with reference to the value of the variable state whichresults from the closest sample to the time of the process event.
 4. Amethod for event-related determination of process variables in controlassemblies comprising detecting process variable profiles by cyclicsampling, and processing said variable profiles in a control program,wherein a process event time is related to a cyclic sampling time, andwherein the control program determines an instantaneous value, whichcorresponds to the time of the process event, of a process variable bycalculation via a ratio assessment, which takes account of the relativeposition of the time of the process event, from the instantaneous valuesof the process variable at the time of the samples on either side of theprocess event.
 5. The method according to claims 1 and 4, wherein theprocess variable is detected using components which are coupled by meansof a bus system.